Imagine you have some notes in which you are indenting to signify some information, like this:

Now click on the Level 1 line (either one). If you now hit ALT+SHIFT+2, all the content under Level 2 gets collapsed and OneNote shows only the content at Level 2 or above. Here's what you see:

With a little plus sign to show you there is more content that is below that level and hidden. From here you can hit ALT+SHIFT+3 to show all the content at level 3 or higher and so on for all the levels you have. ALT+SHIFT+0 expands all levels.

I explained this to a few folks internally and thought it may be worth passing on. If you are a habitual users of indentation to help organize your notes, these keyboard shortcuts might save you some time when reviewing them.